Using T cell recognition (primarily cell-mediated lysis) we have continued to probe the complexities of T cell recognition of HLA alloantigens. We have pursued the novel approach of using cytotoxic T lymphocyte (CTL) clones as the selective agent to enrich for ICR-191 mutagenized lymphoblastoid cell lines (LCL) which have lost their capacity to be lysed by that clone. Using that approach, 24 mutant LCL have been derived which are not lysed by the "selecting" DPw2-allospecific CTL clone. Serological analysis confirms the loss of DPw2 and Nothern blot analysis demonstrates loss of DP alpha or DP beta mRNA in many of the mutants. These mutants have subsequently been used as informative probes to demonstrated the requirement for DP alpha and DP beta recognition by other putatively "DPw2-specific" CTL clones with more complex specificity. The specificity of these clones suggests that they may recognize peptide fragments of another HLA gene(s) in a DP-restricted fashion.